Antistatic layers for polymeric photographic supports

ABSTRACT

Photographic elements having polyolefin supports coated with silver halide emulsions highly sensitive to static discharges, are protected against static by coating a polyolefin surface with a mixture of a polyelectrolyte, a polyalkylene oxide and colloidal silica. The static protection obtained is substantially better then obtained by coating the polyolefin surface with any one or two of the three materials.

United States Patent Inventors Douglas C. Joseph Victor; William C. Kerr, Middlesex; Harold K. Reed, Rochester, all of N.Y. Appl. No. 869,086 Filed Oct. 24, 1969 Patented Dec. 28, 1971 Assignee Eastman Kodak Company Rochester, N.Y.

ANTISTATIC LAYERS FOR POLYMERIC PHOTOGRAPHIC SUPPORTS 7 Claims, 1 Drawing Fig.

U.S. 96/85, 96/87 A, 96/114.2, 117/76 F, 117176 P, 161/250, 161/252, 260/D1G. 15, 260/D1G. 19

1nt.C1 G03cl/82, G03c1/86 Field olSearch 96/85,87

A, 114.2; 260/D1G. 15,D1G. 19, 17 R, 18 R, 19 R, 20; 117/76 F, 76 P, 138.8 E; 162/250, 252

References Clted UNITED STATES PATENTS Myers Morey et Mackey et a1. Schleede et a1. Sterman et al Nadeau et a1.

Walker Magat et a1. Nadeau Miller FOREIGN PATENTS Canada Primary Examiner-John T. Goolkasian Assistant ExaminerGeorge W. Moxon, l1 Attorneys-Walter 0. l-lodsdon and Wendell H. McDowell ABSTRACT: Photographic elements having polyolefin supports coated with silver halide emulsions highly sensitive to static discharges, are protected against static by coating a polyolefin surface with a mixture of a polyelectrolyte, a polyalkylene oxide and colloidal silica. The static protection obtained is substantially better then obtained by coating the polyolefin surface with any one or two of the three materials.

EMULSION POL Y 0L E F //V PAPER POLYOLEF/IV ANT/STATIC LAYER 0F COLLO/DAL SILICA, POLYALKYLENE OXIDE AND ANION/C POLYE LE 6 TROLY TE iAIENH-Illum2ansn 31530,

POL r01. EF/N ANT/STATIC LAYER 0F COLLOID/1L SILICA, POLYALKYLE/VE OXIDE AND ANION/C POLYELECTROLYTE DOUGLAS 6. JOSEPH WILLIAM C KERR HAROLD K. REED INVENTORS WQW 1/ 6 Arron/95y a AGE/V7 ANTISTATIC LAYERS FOR POLYMERIC PHOTOGRAPHIC SUPPORTS This invention relates to the protection of photographic elements having polymeric supports against static.

lt is known that paper for photographic use may be rendered substantially waterproof by application thereto of various hydrophobic polymeric materials such as styrene polymers, polyacrylates, polyethylene, etc. Although such photographic papers show markedly improved water resistance, they have not proven entirely satisfactory because of their tendency to accumulate static electrical charges during manufacture, handling and use. The static discharges cause irregular fog patterns in the photographic emulsion coated thereon. The static charges are also undesirable because dirt, which the charges attract to the web, causes repellancy spots, desensitization, fog and physical defects. On polymer coated papers such as polyethylene coated papers this static problem is serious because of the nonconductive nature of the polymer coating. However, since the polyolefins have other outstanding properties as supports for photographic silver halide emulsion layers it has been desirable to find antistatic coatings which are as effective as possible.

A wide variety of materials have been recommended for use in antistatic coatings for photographic film supports. These materials have included inorganic salts and salts of polymers such as polymeric carboxylic acid salts. More recently, French Pat. No. 1,556,240, Dec. 30, 1968, recommended the use of antistatic coatings of a mixture of colloidal silica and salts of polymeric carboxylic acids for application to polyolefin surfaces. Thus, the surface resistivity of polyethylene surface is lowered from about 16 log ohms to about 12 log ohms (measured at 70 F. and 20 percent RH). These silica-containing coatings are very effective in protecting silver halide emulsions of average light-sensitivity against static, but do not give the desired static protection for the higher speed silver halide emulsions where a surface resistivity of less than about log ohms is required.

We have discovered that polyolefin photographic supports can be coated with certain compositions to reduce the surface resistivity sufficiently below 11 log ohms, that very high speed emulsions coated thereon are not affected by any low level static discharges occurring in handling the product. The antistatic coatings we use for this purpose comprise an aqueous mixture of 1 a film-forming anionic polyelectrolyte such as a polymeric carboxylic acid, (2) a polyalkylene oxide and (3) colloidal silica. The proportions of the three components of the antistatic compositions should be carefully controlled for optimum results as the data below will show. Also, the data show that coatings of these three-component compositions are substantially more effective for static protection than coatings of any one of a mixture of any two of the three components.

The static charges under consideration in the present invention occur on photographic sensitizing machines or high speed slitting and spooling equipment in two types, namely, unwind static and transport static. A corona forms at the unwind stand where the unwinding paper separates from the roll; this is called unwind static, and is affected by speed, the composition of the emulsion coating and the wire side coating. Static discharges also occur throughout the machine as a result of contact and separation of the coatings with transport rollers; transport static is affected by speed, the composition of the rollers and wire side surface. The internal conductivity and moisture content of the sheet and the atmospheric conditions of the room also affect static generation.

In other words, the generation of the static charge is a dynamic phenomena which is affected by the rate of contact electrification of the sheet by friction and the conductivity of the sheet (surface and internal) which controls the rate of dissipation of the charge. The two factors, electrification and dissipation, must be correctly balanced or a corona forms at the unwind and spark discharges occur as transport static. To avoid static the dissipation rate must be greater than the electrification rate. This value is determined herein by measuring surface conductivity in terms of surface resistivity at specific conditions of temperature and humidity, namely, at 75 F. and 20 percent relative humidity, between two electrode plates and calculating the surface resistivity according to the formula:

Resisitivity (ohms)=Resistance observed (ohms) Spacing between electrodes (ems) aenstks e w s (we? For further details of the above method for measuring surface resistivities, reference may be had to G. F. Nadeau et al., U.S. Pat. No. 2,801,191, issued July 30, 1957.

In accordance with the invention, the improved light-sensitive photographic papers or elements are prepared by applying a coating of a polyolefin such as polypropylene or polyethylene onto each side of the paper stock by solvent coating methods, by hot melt extrusion or by lamination of a preformed sheet of the polymer thereto, as is well known in the art. Different polyolefins may be coated on each surface. Ordinary photographic paper stock can be employed for coating with the polyolefin, or the stock may be tub sized, as described by U.S. Pat. No. 3,253,922, with a solution ofa conducting salt, e.g. an alkali metal sulfate such as sodium sulfate, which acts as an internal antistat in direct contact with the paper (i.e. an antistatic agent between polyolefin layers 11 and 12 of FIG. 1) and provides useful antistatic properties for the final photographic product. However, there is evidence that when the internal antistat is present and corona activation of the surface of polyolefin layer 11 is used to effect the adhesion of emulsion layer 14, the internal antistat causes irregular activation of layer 11. When our antistatic layer 13 of components (l (2) and (3) is used, unexpectedly more satisfactory activation of the polyolefin is obtained.

If desired, the paper stock per se can be subjected to electron bombardment, i.e. corona discharge treatment, prior to the application of any of the polyolefin coatings to improve adhesion. In place of electron bombardment, a primer or subcoating can be used to improve adhesion of the polyolefin to the paper stock. The thickness of the paper stock can vary widely from thin to semin'gid sheets as desired. The thickness of the polymer layers such as polyethylene layers can also vary over a wide range depending on the requirements of the final photographic product. Polyethylenes and polypropylenes capable of forming a continuous film can be used in the above procedure.

The polyolefins are extruded on paper so as to obtain about two to eight pounds per 1,000 sq. ft. on each surface. The polyolefins used in the process are the aliphatic polyolefins, polyethylene, polypropylene and copolymers of ethylene and propylene. Useful polyethylenes have a density range of about 0.910 to 0.980 g./cc., their viscosity measured by Melt lndex (ASTM D4238, condition E) can be about 2.0 to 20.0, preferably 3.0 to 12.0 decigrams per minute and they can be about 40 to 90 percent crystalline. Useful polypropylenes have a density range of about 0.900 to 0.910, their viscosity measured by Melt Flow Rate (ASTM D-1238, condition L") can be about 10 to 90, preferably 40 to 90, decigrams per minute and they can be about 60 to percent crystalline. The polyolefins are prepared by well-known methods of polymerization of ethylene and propylene using, for example, Zeigler catalyst. Blends of high density polyolefins and low density polyolefins (the latter originating from either initial polymerization of olefins to lower density, or by heat degradation of high density polymer) can be employed. Other aliphatic polyolefins which are useful are the polyallomers, i.e. copolymers of ethylene and propylene prepared, e.g., as described in the Hagemeyer U.S. Pat. applications Ser. Nos. 505,227 filed Oct. 26, 1965, 516,783, now U.S. Pat. No. 3,478,128 and 516,677, now abandoned filed Dec. 27, 1965. Similarly, the antistatic compositions are applied to clear or pigmented polyolefin sheeting or foils for static protection.

The mentioned polyolefin coated papers or sheetings can be used not only as photographic emulsion supports but also in the other arts wherein static presents a problem.

The polyelectrolytes l) of the antistatic layers should be of the water-soluble anionic film-forming type. The following are representative: alkali metal salts or ammonium salts of polymeric carboxylic acids, e.g. polymethacrylic acid sodium salt; such salts of cellulose sulfate; such salts of polyvinyl phosphate or such salts of a partially esterified lactone of a vinyl alcohol-a,fi-dicarboxylic acid copolymer. The latter salts are prepared from lactones of resinous heteropolymers of vinyl alcohol and unsaturated a,,B-dicarboxylic acids, optionally esterified with monohydric alcohols and prepared as described by U.S. Pat. No. 3,260,706 and converted to the alkali metal salts as described in U.S. Pat. No. 3,169,946. Representative of these ester-lactone salts is the butyl lactone salt of example 1 of U.S. Pat. No. 3,260,706 converted to the sodium salt as described in example 17 of U.S. Pat. No. 3,169,946.

The polyalkylene oxide component (2) of the antistatic coatings are critical particularly with respect to molecular weight, as data of the examples below will indicate. That is, while the alkylene units of the carbon chain may contain two to four carbon atoms as present in polyethylene, polypropylene and polybutylene oxides and blends thereof, the molecular weight is selected so optimum antistatic protection is obtained. For example, the molecular weight should be about 400 to 1,000 when polyethylene oxides are used, but the optimum molecular weight may be expected to vary somewhat above or below this range depending upon the particular oxide used.

The colloidal silica component (3) of the antistatic coatings is well known in the art and can be prepared as described in U.S. Pat. No. 2,701,218 or readily obtainable under the trade name Ludox AM (an aluminum modified colloidal silica) from the E. l. duPont de Nemours and Co.

The proportions of components (1), (2) and (3) of the antistatic coatings of the invention are critical to obtain the optimum level of surface resistivity, i.e. less than about 11 log ohms.

The antistatic coating applied to the polyolefin surface contains the ingredients (1), (2) and (3) above in proportions such that the optimum static protection is obtained and the coating is substantially free of tackiness. For these purposes the coatings should contain approximately the following proportions by weight:

(l) Polyelectrolytc (2) Polyalkylene oxide The balance of the antistatic coating is essentially colloidal silica (3) which is used in an amount necessary to prevent tackiness of the coating and concomitant transfer to other surfaces. For this purpose, the coatings contain about 50 to 80 percent of colloidal silica. While as much as 80 percent of component (3) can be used with 4 percent of (l) and 16 percent of (2), this coating gives less desirable static protection. Accordingly, it is preferred to use less silica and more of (l) and (2). Similarly, it is less desirable to use the lower levels of about 50 percent of colloidal silica with the higher level of about 30 to 35 percent of (2) since the coating tends to be tacky. Accordingly, a very useful antistatic composition contains about 4 percent polyelectrolyte such as polymethacrylic acid salt, about 30 percent polyalkylene oxide such as polyethylene oxide molecular weight 400 and about 66 percent colloidal silica.

The antistatic coatings containing materials l (2) and (3) above are preferably coated upon the polyolefin surface after activation of the surface with chemical agents such as acid dichromate solution, or after flaming, but preferably after ac tivating the polyolefin surface with corona discharge. The corona applied to the polymer surfaces is supplied by wellknown power sources. The spark gap type power source for the corona has current supplied to the electrodes by a spark gap excited oscillator in a well-known manner. Variation in fundamental frequency of the corona is obtained by changing the primary power frequency of the oscillator in a range up to 10,000 or more cycles per second. A high-voltage corona is desirable, e.g., 25,000 to 50,000 peak volts or higher, to obtain adequate adhesion of the layers to the corona activated surface. Voltages of this range are adequate for corona activation of polymers at web speeds of about to 1,000 feet per minute or higher.

The corona can be applied to the polymeric surface, for example, by means of several metal electrodes positioned close to the polymeric surface at a point where the polymeric surface is passing over a grounded metal roll coated with a dielectric metal such as a linear polyester. Similarly, a metal roller may be used to support the web with the other electrode array being in planetary disposition equidistant from the surface of the metal roller and each being coated with a dielectric, at least on the surface nearest the metal roller. The spacing of the electrodes to the polymer surface and ground roll should be adequate to produce the corona at the voltage used and yet allow for free passage of polymeric sheet through the activating zone. Corona supplied by AC current, or a combination of AC superimposed on DC can be used.

The accompanying drawing shows in greatly enlarged crosssectional view the appearance of a representative photographic element of our invention wherein the polyolefin layers 11 and 12 are adhered to the photographic base 10 and emulsion layer 14 such as gelatin-silver halide emulsion is adhered to polyolefin layer 11. The polyolefin layer 12 is provided with a thin coating of the antistatic layer of the invention comprising components (1), (2) and (3), i.e. polyelectrolyte, polyalkylene oxide and colloidal silica.

The following examples are representative of the processes and materials useful for carrying out our invention.

Example 1 A photographic paper stock of about 22 pounds per 1,000 sq. ft. is supplied. Each side of the stock is activated with corona discharge and extrusion coated with polyethylene at about 8 pounds per 1,000 sq. ft. One polyethylene surface is then corona activated and coated in-line with the following composition so as to obtain 0.5 to 2.5 grams, preferably 1.5 grams, solids per square meter.

Colloidal silica (30% dispersion) 22.0 g Sodium cellulose sulfate 0.4 g Polyethylene oxide (mol. wt. 400) 3.0 g. Water 74.6 g.

containing approximately 2.5 sulfate units per anhydroglucose unit (sold as such by the Kelco Chemical Co.). Other water-soluble sodium cellulose sulfates known in the art can also be used.

This composition containing 66 percent colloidal silica, 4 percent sodium cellulose sulfate and 30 percent of the oxide (dry weight basis) represents a preferred proportion of the ingredients, sufficient silica being present to obtain a nontacky surface. The surface resistivity of the coated paper measured at 70 F. and 20 percent RH is about 9.7 log ohms compared to 16 log ohms for the polyethylene coated paper free of the antistatic layer.

The other polyethylene surface of the paper is then corona activated and a high-speed gelatin-course-grained silver bromoiodide emulsion is coated. It is found that the emulsion is not affected by static charges due to handling on the emulsion coating machine, whereas the same paper free of the antistatic backing layer produced static discharge patterns in the emulsion layer.

Similar surface resistivities are obtainable when polyethylene sheeting is used in place of the polyethylene coated paper stock.

Example 2 The process of example 1 is repeated except using the following polyalkylene oxides with the results shown. The coatings were dried at room temperature.

Molecular Weight Polyethylene oxide 200 5.8 Polyethylene oxide 400 9.1 Polyethylene oxide L000 l0.3 Polyethylene oxide 6,000 l2.5 Polypropylene oxide 400 I06 These data show that while polyalkylene oxides of 200 to 1,000 molecular weight provide substantial static protection, the 200 to 400 molecular weight range materials should be used where the emulsion requires a coating surface of resistivity less than 10 log ohms. 7

Similar tests using glycerine, triethylene glycol and diethylene glycol with cellulose sulfate and colloidal silica in place of the polyalkylene oxides show that while initially surface resistivities in the range of 8.8 to 12 log ohms areobtained on the room temperature dried samples, upon drying the paper at 200 F. to simulate conditions on emulsion coating machines, the resistivities were generally unsatisfactory. Under the same 200 F. drying conditions the resistivities of the 400 to 1,000 molecular weight polyethylene oxide samples were substantially unchanged. Example 3 The process of example 1 is carried out except that corona activated surface is coated with the same amounts (dry weight basis) of one or more of the three components of the composition with the results shown.

Coatings too tacky to be useful.

Sample (f) illustrates the unexpected effectiveness of the combination of colloidal silica, polyelectrolyte and polyalkylene oxide to reduce static and produce nontacky coatings.

The above examples illustrate polyolefin photographic supports carrying the antistatic layers of the invention primarily for protection of high-speed black-and-white photographic emulsions. The same supports can be coated with a plurality of well-known differently sensitized color emulsion layers to provide products for color photography. Since these color emulsions have relatively low speeds the surface resistivity need not be as low and the polyolefins in the 400 to L000 molecular weight range become more generally useful.

The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

We claim:

1. A photographic support having a polyolefin surface and an antistatic coating thereon comprising a mixture of:

I. about 4 to 25 percent by weight of an anionic film-forming polyelectrolyte,

2. about 16 to 35 percent by weight ofa polyalkylene oxide of molecular weight of about 400 to l ,000, and

3. about 50-80 percent by weight of colloidal silica.

2. The support according to claim 1 wherein the polyolefin surface is modified by corona discharge for adhesion of the antistatic coating.

3. The photographic support according to claim 1 wherein the support is paper having polyolefin extrusion on each side and the antistatic coating is on one of the polyolefin surfaces.

4. The photographic support according to claim 2 wherein the polyolefin surface comprises polyethylene or polypropylene or a blend thereof, or a copolymer of ethylene and propylene.

5. The photographic support according to claim 4 wherein the polyalkylene oxide is a polyethylene oxide.

6. The photographic support according to claim 4 wherein the surface carrying the antistatic coating is polyethylene and the polyelectrolyte of the antistatic coating is an alkali metal or ammonium salt of either polymethacrylic acid, cellulose sulfate, polyvinyl phosphate or a partially esterified lactone of a vinyl alcohol-a,B-dicarboxylic acid copolymer.

7. The element of claim 1 coated with at least one light-sensitive photographic emulsion layer. 

2. about 16 to 35 percent by weight of a polyalkylene oxide of molecular weight of about 400 to 1,000, and
 2. The support according to claim 1 wherein the polyolefin surface is modified by corona discharge for adhesion of the antistatic coating.
 3. about 50-80 percent by weight of colloidal silica.
 3. The photographic support according to claim 1 wherein the support is paper having polyolefin extrusion on each side and the antistatic coating is on one of the polyolefin surfaces.
 4. The photographic support according to claim 2 wherein the polyolefin surface comprises polyethylene or polypropylene or a blend thereof, or a copolymer of ethylene and propylene.
 5. The photographic support according to claim 4 wherein the polyalkylene oxide is a polyethylene oxide.
 6. The photographic support according to claim 4 wherein the surface carrying the antistatic coating is polyethylene and the polyelectrolyte of the antistatic coating is an alkali metal or ammonium salt of either polymethacrylic acid, cellulose sulfate, polyvinyl phosphate or a partially esterified lactone of a vinyl alcohol- Alpha , Beta -dicarboxylic acid copolymer.
 7. The element of claim 1 coated with at least one light-sensitive photographic emulsion layer. 